Tube rolling process



y 1936 K. KORBULY 2,041,937

I TUBE ROLLING PROCESS Filed Oct. 25, 1955 May 26, 1936- K. KORBULY 2,041,937

TUBE ROLLING PROCES S Filed 001;. 23, 1955 Patented May 26, 1936 UNITED STATES PATENT OFFICE TUBE ROLLING PROCESS Application October 23, 1935, Serial No. 46,402 In Austria October 27, 1934 Attempts have often been made to make seamless metal tubes by drawing or rolling on a mandrel from a blank in one heat with a small wall thickness, and with a reduction in the crosssectional area of the material of the blank down to /15 and at least but heretofore it has not been possible to obtain useful results in the case of hard metals, such as iron.

Since such a considerable reduction in the cross-section requires a comparatively large number of drawing tools arranged in series, the said number exceeding the denominator of the fraction representing the reduction in cross-section, the stresses produced in the material arevery considerable. The high frictional resistancewhich occurs in the usual rigid passes and which has to be overcome by the pressure of the drawing or forcing mandrel acting upon the bottom of the tube-blank should not attain the tensile strength of the material in the smallest crosssection of the blank, that is to say, in the vicinity of the bottom of the blank, and thereby a narrow limit is set to the maximumreduction in crosssection attainable in one working operation.

In order to minimize the friction, it is known to construct the drawing passes of loose rolls, but in this case also the blank on the mandrel had to pass through only one drawing stage in one Working operation, so that a thin-walled tube can only be made by repeated treatment on the drawing bench. v

It is also known to pass the blank in one working operation on the same drawing bench successively through two drawing passes formed by 1 loose rollers, but in this case the second pass is of a shape to flatten only the ribs produced in the first pass, and to loosen the tube on the mandrel without any further reduction of its wall thickness.

According to another known proposal, in order to reduce the resistance of the drawing passes formed by the rolls, the rolls of the passes are set in rotation by the fact that the drawing passes driven by the blank roll on tracks on the drawing bench. Since however, the peripheral speed of the rolls is dependent upon the feed of the blank, the differences in the internal diameters of the various passes may onlylbe small. It is also not possible in this way to attain any considerable reduction in cross-sectionin one working operation. 1

It is further known to drive the rolls of the drawing passes through which the blank is drawn by the mandrel, positively by means of a gear wheel mechanism, so that the feeding of the 2 Claims. (Cl. 80-62) blank is effected both by the driven mandrel and by the driven rolls of the drawing passes, on the assumption that'it is thereby possible to obtain a much greater reduction in one operation than previously. In this case, however, the pressure 5 produced in the pass is so high that the metal is forced deeply into the gaps between the rolls, and the resulting rib formation gives .rise to fresh diificulties. A number of various proposals have been made to remove these diificulties. It was then recognized that it is impossible to efiect a considerable reduction in cross-section in one drawing pass, even if the pass is composed of rolls of the smallest possible diameter and both the rolls and the mandrel are driven positively, but that it is necessary to arrange a relatively large number of drawing passes in series the positive drive of such passes being,-however, retained. In this case, however, there is the difiiculty of having to drive the rolls of difierent drawing passes at different speeds, which latter have to be adapted to the size of each particular pass and the degree of reduction occurring in the various passes, so that special change speed gears would have to be incorporated for each of the large number of drawing passes to be employed, which would result in very considerable constructional difliculties rendering the practical application of this proposal impossible.

Attempts have therefore been made to dispense with the positive drive of the rolls of the drawing passes. According to a known proposal in this direction, the rolls of every alternate drawing pass are provided with a positive drive whilst the rolls of the intermediate passes are driven by the blank.

Finally, there are also entirely non-driven rolls in a plurality of drawing passes arranged in series. The arrangement according to this proposal, however, is such that during the main drawing operation the blank is forced from a closed cylinder by a special plunger, after the manner of.an extrusion press through the pass and around the-mandrel which is stationary relatively to the forwardly-driven blank. In addition, the outer surfaces of the rolls are supported on stationary drawing rings, so that they are unable to move freely during pressing, that is to say, the friction in the drawing pass will not be a rolling one. In addition the ratios of the roll diameters to the pass apertures are not selected drawing seamless tubes of hard metal, particularly i ron, starting from the tube blank, down to the usual wall-thickness of gas piping in one working operation by forcing the glowing blank carried by the mandrel through a large number of drawing passes disposed in series and formed by non-driven rolls. Since in this case the force required for drawing is transmitted to the blank solely through the mandrel applied against the bottom of the tube blank, then-if the total reduction in cross-section to be effected in one working operation is so great that the force necessary for this purpose, together with that required for overcoming the frictional resistance of the drawing passes, would produce in the smallest cross-section of the blank a stress ex:- ceeding the tensile strength-rupture of the blank in the dangerous cross-section would necessarily occur. The stress in the dangerous cross-section cannot be diminished in practice to any desired extent by increasing the distances at which the drawing passes follow one another so far that only one or only a few of the large number of drawing passes required for effecting the desired total reduction in cross-section, act simultaneously on the blank at any given instant. In this case, the drawing bench would have to be so long as to give rise to difficulties in regard to its construction. In addition, since the speed of the blank cannot be increased to any desired extent, the blank would cool during the time required for passing through the entire series of drawing passes, below the temperature necessary for the treatment before the completion of the working. The means for reducing the resistance opposed to the passage of the blank through the passes by constructing the drawing passes of rolls of the smallest possible size is not in itself sufficient to produce such a considerable reduction of the stress in the dangerous cross-section of the blank that it would be possible to discard the external drive of the rolls or of some of them.

In order to obviate entirely the use of driven rolls it has now been proposed to provide behind or between the drawing passes, wherein rolls in the .narrowest cross-section have at the most twice the diameterofthe corresponding pass, also passes which produce an increased friction by the fact that the diameter of the rolls forming these passes is greater than that of the other rolls and in the narrowest cross-section amounts to from 1.5 to 4 times'the diameter of the pass.

It has now'been found that this object is not achieved with all arrangements of passes wherein the roll diameters are below the'above-mentioned limit value of at the most twice the diameterof the pass at the largest pass, and wherein for some passes, the friction passes, the roll diameters are of the pass, and that the sequence in whichthe rolls of different diameters follow one another is not a matter of indifference.

The difficulties produced by the tensile-stresses exerted on the blank and the necessity'for taking special steps to diminish these stresses come of course into consideration only when it is necesbetween 1.5 and 4 times the diameter be performed with the smallest possible number of passes, that is to say, each pass effects a re.- duction of about 20% to 25% relatively to the entrance cross-section of the material in the respective pass.

For a total reduction to /5 of the cross-section of the material it will be necessary to have at least 6 roll passes arranged in series, for a reduction to /16 at least 12 and for a reduction to /15 at least 15, i. e. a number of roll passes at least equal to the reduction factor will be necessary.

According to the invention, it is important that the roll diameter measured at the bottom of the groove, that is to say, the trough diameter of the rolls in the largest pass should be at least 0.3, and in the smallest pass should be at the most 2.5 times the corresponding internal diameter of the pass, and that the ratio between the roll diameter and pass diameter increases gradually from the largest pass to the smallest pass.

In making tubes of a diameter below about millimetres and having the usual wall-thickness of gas piping, about 2-3 millimetres, the roll diameter, which within the above mentioned limits is smaller at the largest pass than the internal diameter of the pass, increases according to the invention gradually towards the smallest pass to exceed the internal diameter of this pass. Preferably the actual size of the rolls of the smallest pass, within the value D being the internal diameter of the pass and d the trough diameter of the rolls of this pass, will be made as small as possible but large enough for the ratio to be diminished in stages which must satisfy the requirements set forth hereinafter, towards the largest, that is to say the first, drawing pass, without reaching the limit value (1 being only 0.3

with fifteen passes disposed in series, in which,

in the smallest, i. e. the fifteenth pass 11 D15 and in the first, i. e. the largest pass d, 'D10.72 v The choice of the size of the diameter of the rolls of the several passes or the graduation of thediminution of the roll diameters are based on the following considerations explained with reference .to the accompanying drawing.-

Figure 1,.is adiagrammatical longitudinal section through the passes of a drawing bench.

Figure 2 is across-section through one'of the passes on a larger scale.

Figure 3 is a part front elevation of a pass on a still larger scale.

The mandrel b of diameter do carrying the blank a which is closed at one end 0 and has the initial diameter Do is forced with the speed V inthe direction of the arrow through the passes K1: to Kn arranged in seriesone after the other.- The original cross-section oi. the blank is to be reduced to the cross-section In Figure 1 only the first and the last three passes are shown, but the number of passes will be from 6 to 16 according to the reduction. 7

The passes K1 to Kn are composed of loose, that is to say non driven, rolls which enclose the blank as completely as possible in their grooves. The grooves of the rolls follow, however, preferably, as shown in Figure 3, only up to an angle of 30" at either side of the middle plane of the rolls the circular periphery of the cross-section of the work-piece and deviate slightly from this latter towards the ends of the rolls. The roll diameter d is the diameter measured at the bottom of the groove, that is to say, the smallest or trough diameter of the roll. The length wo of the blank extending from the cross section nnthe pass Kn to the cross-section Qn of the material, with which the blank leaves the said pass, the vertical pressure on the blank and hence the pressure with which the material is applied against the mandrel can be increased, the friction between the blank and the mandrel can thereby be increased to such an extent that the bottom of the work -piece is partly relieved of the pressure of the forcing mandrel, and hence only a part of the tensile stress of the material is transmitted from its bottom to the cross-section n-n. Due to the reduction produced by the pass Kn, the material flows backwardly at the speed Sn opposite to the direction of feed of must have a smaller diameter relatively to the diameter IDs-1 than in the last pass, that is to iii) say

3?; n-l must be less than Hence an upper limit is thereby set to the diameter of the rolls in the penultimate pass. The lower'limit is determined by the preceding passes lKn-a to K1 firstly by the iact that the rolls of all the preceding passes have to satisfy the requirement that the ratio up to the first pass K1 should not fall below 0.5 or in the extreme case below 0.3. Secondly, the roll diameter isl'imited downwardly by the fact that though the frictional force between the material and the mandrel must not be so great as to prevent a backward flow at the speed sn, it should nevertheless be as great as possible below this limit in order to relieve the blank partially of the tensile stress produced by the forcing mandrel.

In the length 102 between the passes Kn-l. and Kn-2, the material will owing to the reduction produced by the pass Kn1 be forced backward at the speed 8nl, so that the material moves under the rolls of the pass Kn-2 in the direction of feed at a speed equal to the diiierence of the speed of feed V of the mandrel and of the sum of speeds 8n+8n-l. The diameter (in-*2 of the rolls of the pass Kn2, must consequently be made small enough that the friction produced by the pressure with which the blank is applied against the mandrel may not hinder the backward flow of the material and that the rolls of the pass may be able to rotate with the peripheral In view however of the following larger pass in the backward direction 1. e. opposite to the direction of feed, the rolls must be made as large as possible in order to relieve partially, owing to the friction caused by the application of the material against the mandrel, the material from the tensile stress, and in order to render possible a further diminution of l the ratio r towards the rearwardly following larger passes up to the first pass K1 without going below the lower limit of this ratio.

Despite the diminution in stages of the ratio d d 5 from 1 but 2.5 at the most, to

roll diameter does not undergo any excessive diminution, because of course the value D increases in stages from the smallestpass Kn to 4 the largest pass K1. Another circumstance which renders it possible not to have to diminish excessively the roll diameter relatively to the rolls of the next smaller pass is that the admissible radial pressure in the pass may be greater for a greater thickness of the layer of material than for a thinner layer of material without checking the backward how of the material under therrolls to such an extent as to produce a jumping up of the layer of material fed to the next smaller pass. The increase in the pressure or application towards the mandrel with increasing thickness of the layer oi material is, however, also necessary in order to produce through a greater wall-thickness the desired friction between the material and mandrel.

Due to the fact that, for the smallest pass Kn, the roll diameter is not made so great that the pressure oiijithe roll forces the material into the gaps between the rolls, and the ratio between the roll diameter and pass diameter is diminished in stages towards the larger passes, there will not be produced in any of the passes such a high pressure that the material pressed into the gaps between the rolls would form ribs so pronounced that they would not be evened out in the next 40 but not below 0.3, the absolute magnitude of thepass without over-rolling. On the other hand,

however, the pressure ofapplication between the material'and the forcing mandrel is suihcient that the tensile stress produced in the material by the force necessary for the reduction from the original cross-section to the final cross-section and imparted to the blank by the mandrel, may be distributed among all the passes, so that a stress exceeding the 'ultimate strength of the material cannot occur in any cross-section of the blank. For the absolute magnitude of the diameter of the rolls of a pass, however there is a tolerance corresponding to the force necessary for accumulating the material in the length lying between the pass under consideration and the next smaller pass. Within this tolerance. if

there are a sufliciently large number of passes, 7

it is possible in the individual passes to depart from the optimum value of the ratio passes which effect an actual stretching or reduction and do not serve other purposes, such as for instance for smoothing the tube or loosening it from the mandrel.

' What I claim is:-- Y

1. A process for the manufacture of seamless metal tubes in which the blank, mounted on a mandrel is reduced by means of reducing passes, consisting of loose rollers, in one operation to at least A, of the cross-section of the blank, char: acterized in that reduction takes place in passes disposed in series and in a number at least equal to the reduction factor, in which passes the ratio between the roll diameter and pass diameter increases successively from the largest reducing pass to the smallest reducing pass and characterized in that the trough diameter of the rolls in the largest reducing pass is at least 0.3 times and in the smallest reducing pass is at the most 2.5 timesthe internal diameter of the respective pass.

2. A process as claimed in claim 1, characterized in that reduction takes place between loose rollers, the trough diameter of which, within the limits specified in claim 1, is smaller in the largest pass than the internal diameter of the latter, and increase successively towards the smallest reducing pass to a magnitude lying above the internal diameter of this last-mentioned pass.

- KARQLY KORBULY. 

